DOCUMENT RESUME

ED 042 603 SE 008 164

AUTHOR Dhabanandana, SalagTITLE UNESCO Chemistry Teaching Project in Asia:

Experiments on Nuclear Science.INSTITUTION United Nations Educational, Scientific, and Cultural

Organization, Bangkok (Thailand).PUB DATE 69NOTE 42p.

EDRS PRICE EDRS Price MF-$0.25 HC-$2.20DESCRIPTORS *Chemistry, Foreign Countries, Laboratory

Techniques, *Nuclear Physics, Radiation Effects,*Radioisotopes, Science Activities, *SecondarySchool Science, *Teaching Guides

IDENTIFIERS UNESCO

ABSTRACTThis teacher's guide on nuclear science is divided

into two parts. The first part is a discussion of some of theconcepts in nuclear chemistry including radioactivity, types ofdisintegration, radioactive decay and growth, and tracer techniques.The relevant experiments involving the use of radioisotopes arepresented in the second part. The experiments include the use ofGeiger counters, investigations of the properties of radiations, andseparation techniques such as paper chromatography, ion exchange,solvent extraction and precipitation. A simple circuit diagram of aGeiger counter is included. This handbook is intended primarily forteachers of secondary school science in Asia. (LC)

se:004=644W

UNESCO

CHEMISTRY TEACHING PROJECT

IN. ASIAUS DIPillmtot Of MitiM, IDIKifiOlf I WIlflif

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DO(11)1111 MIS IIIN frIPPOOVID ifittif AS RICIIY1D 15014 151

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Sfe Iff 00 501 111CISSatftv 011Kli1 OfIKI Of EDUCATION

POSIIION OS tOtity

EXPERIMENTS ON

NUCLEAR SCIENCE

1989

BANGKOK, THAILAND

The Unesco Project for Chemistry Teaching in Asia

Project Directors

L.E. Strong, (U.S.A.) 1965-1966M. Shafquat H. Siddiqi (Pakistan) 1965

E.C. Watton (Australia) 1966-19671. Zyka (Czechoslovakia) 1968

Present Staff

Prof. J. Zyka, Unesco Expert, Project DirectorDr. Sunt Techakumpuch, Project Co-director, Chulatongkorn University, Br ngkokDr. 11. Herm, Unesco Associate Exp. -t, Project Associate DirectorMr. M.A. Rawoof, Project ManagerMiss Penpan Ohmarak, Miss Kwang Soon Lee, Miss Kaysara Aegkittinate,Miss Matapa Boonruksa, Secretary and Typists

Project Location

Chulatongkorn University, Chemical Technology Building, Phya Thai Road,Bangkok. Thailand

Mailing Address

P.O. Box 1425, Bangkok, Thailand

This Project is an activity of Unesco ( United Nations Educational

Scientific and Cultural Organization), Division of Science Teaching, Place deFontenoy, Paris 7e, France. The Division of Science Teaching Director isDr. H.A. Foecke. The Project Officer in this Division is Dr. R.H. Maybury.

Unesco maintains regional offices to provide assistance to the countriesin a region in a more direct manner. The Unesco regional offices in Asiaare:

Unesco Regional Office for Education in Asia, P.O. Box 1425, Bangkok, Thailand

Unesco Field Science Office for Southeast Asia, Djalan Imam Bondjol 30,Trornol Post 273 DKT, Djakarta, indont-ia.

Unesco Field Science Office for South Asia, Unesco House, 40B, Lodi Estate,New Delhi 3, India.

UNESCO

FEB 5 1970

CHEMISTRY TEACHING PROJECT

IN ASIA

EXPERIMENTS ON

NUCLEAR SCIENCE

1969

BANGKOK, THAILAND

PREFACE

Thi's handbook is intended as a teacher's guide on nuclearscience. Since this topic is not quite as generally known as othertopics of chemistry, the handbook is therefore divided into twoparts. The introduction to nuclear science covers the normal re-quirements for those who have basic scientific knowledge, but wishto employ radioisotope techniques in their work. The practicalapproach to the use of radioisotopes has been made through varioustopics such as radioactive growth and decay, methods of detectionof radiations, tracer techniques. The relevant experiments arepresented in the second part. Since this handbook is primarilyprepared for the benefit of the science education at the secondaryschool level where apparatus, equipments, chemicals and isotopesare difficult to obtain, the experiments have therefore been de-signed for simplicity but ere adequate in a way of illustration ofthe basic properties of radiations and the applications of radioiso-topes. Teacher' are encouraged to use natural occurring isotopessuch as uranium .nd potassium and also to purchase or even make asimple kind of -ounter. For the latter purpose, a simple circuitdiagram of a 4Kr-counter is included in the second part of the book.The author acknowledges the Unesco Chemistry Teaching Project'sstaff and the Thai Atomic Energy Commission's staff for their tech-nical assistance. The Electronic Section of the Thai Atomic EnergyCommission has succeeded in producing the inexpensive counters.Most of the experiments appeartng in this handbook have been testedusing one of these counters.

The contents in this handbook do not claim to be original orcomplete in any sections; additional reading is advisable for thosewho want to be specialised in the field of nuclear chemistry. Con-structive criticisms on this work are sincerely welcome.

Dr. Salag DhabanandanaDepartment of ChemistryChulalongkorn UniversityBangkok, Thailand

- 11 .

CONTENTS

T. INTRODUCTION TO NUCLEAR CHEMISTRY 1

II. EXPERIMENTAL SECTION 14

A. COUNTERS, STATISTICS OF COUNTING AND SOURCE

PREPARATION 15

A.1

A.2

A.3

Counters

Statistics of Counting

Source Preparation

15

16

18

B. PROPERTIES OF RADIATION 19

B.1 Deflection of Radiations by

Magnetic Field 19

B.2 Ionising Property of Radiation 20

B.3 Absorption 21

C. SEPARATION TECHNIQUES 24

C.1 Paper Chromatography 24

C.2 Ion Exchange 29

C.3 Solvent Extraction 31

C.4 Precipitation 31

PART I

INTRODUCTION TO NUCLEAR CHEMISTRY

INTRODUCTION TO NUCLEAR CHEMISTRY

The main difference between chemical reactions and nuclearreactions is that chemical reactions involve the rearrangement ofelectrons in the orbit outside the atomic nucleus whereas nuclearreactions involve the changes within the nucleus. It is thereforeessential for the study of nuclear science to start from the studyof the nucleus.

1.1 The Nucleus and Radioactivity

An atomic nucleus is the central core of an atom and is com-posed of 2 types of particles, protons and neutrons. These parti-cles possess approximately the same mass (mass of proton = 1.007280a.m.u., mass of neutron u 1.008665 a.m.u.) but the proton has apositive charge whereas the neutron is neutral. It is obviouslyseen that an atomic nucleus carries a positive charge. In thenucleus, particles are bound together by a nuclear force. Eachspecies of nucleus contains a definite number of protons and neu-trons. The nuclear charge is balanced by extra nuclear electrons.These three are the fundamental particles which are constitutionof atoms. There are other sub-atomic particles such as mesons,neutrinos, anti protons etc, but they need not concern us here.We shall use the symbols Z, N and A for an atomic number (the num-ber of protons in a nucleus), a neutron number (the number of neu-trons in a nucleus) and a mass number (the sum of Z and NlArespec-tively. In this article the convention z(chemical symbol) e.g.15P32 will used to indicate the characteristics of nuclei.Three types of nuclides are classified using these 3 numbers ascriteria.

Table I

--------Isotope

Isobar

Isotone

Z A N Exam les

same

different

different

different

same

different

different

different

same

15P31 and

15P32

15P32 and

16S32

1H2 and

2He

3

The chemical properties of an atom are due almost eirelyto the electrons rather than to the nucleus. Hence! from Table Ithe isotopes of a given element have the same nuclear charge andthe same number of extra nuclear electrons. Therefore, they willhave virtually identical chemical properties but slightly differentphysical properties such as mass, density, freezing point. If itwere not for some tiny differences due to the different masses oftheir nuclei, it would be impossible to separate isotopes fromeach other by chemical means.

The sum of the mass of component particles in the nucleus isnearly equal to the actual mass of the nucleus in atomic mass units;the difference is small and it is due to the binding energy forthe nucleons in the nucleus. The binding energy is the energywhich would be required to split the nucleus into its component

PAM MOW-

- 3 -

nucleons and is usually expressed as binding energy per nucleon.

Not all combinations or ratios of protons to neutrons arestable, if there is an excess or deficiency of neutrons, the nu-cleus will spontaneously disintegrate in order to reach a morestable arrangement, in other words the atom will be radioactive.The radioactive atom is unstable, it nucleus undergoes disintegra-tion all the time with characteristic half-life (see later). It

is also characterised by the type and energy of the emitted radia-tion. The process of disintegration is affected by the temperature,pressure or concentration of the reacting substances. There are3 kinds of radiations emitted from the unstable nuclei, alpha(abeta ( 8) and gamma (y ) radiations. These radiations can affectphotographic plate so that they are darkened on development, theycan ionise a gas by ejecting an electron from an atom, of gas andproduce scintillation or small flashes of light in certain sub-stances such as zinc sulphide. The magnetic field can deflectalpha and beta particles but not gamma rays. Alpha particles arethe nuclei of helium atoms travelling at high speed and have a lowPenetrating power (a thin sheet of paper or a few centimetres ofair can stop alpha particles completely). They produce intenseionisation in the few centimetres of air. Beta particles arefast-moving electrons and can penetrate a few millimetres of alumi-nium or about 700 centimetres in air. Gamma rays are electromag-netic radiation like x-ray but have shorter wavelengths or higherfrequencies. They can penetrate long distances through air orseveral centimetres through lead.

1.2 Types of Disintegration

a. Alpha Disintegration. Usually alpha particles are emittedby heavy elements e.g. radium emits alpha particles and decays toradon. Most isotopes emit particles of more than one energy.Magnetic analysis of alpha particles gives a spectrum consistingof one or more sharp lines, each line coploponding to a particularkinetic energy. Suppose two nuclei of (143p decay by elpitting alphaparticles of different energy to form 2 nuRivi of Th234. Accordingto the first law of thermodynamics, the Th4;3.4,,,produced by the emis-sion of the lower energy a-particles from U4." has more energy thanthat obtained from the higher energy particle emission.

The more energetic nucleus is called an excited state of Th234

.

In order to come to a lover state of energy (more stable), Th234must emit the extra energy in the form of an electromagnetic radia-tion, a gamma ray.

b. Beta Disintegration. Ordinary beta decay, as in thenatural radioactive series, is commonly found throughout the rangeof the periodic table. This phenomenon occurs when there is anexcess of neutrons in a nucleus. In order to achieve a stablestate, one neutron splits into a proton and an electron. Only thenegative electron (negatron) with high speed is emitted. Thisparticle is termed TheThe emission of beta particles differsfrom that of alpha particles in respect to the spectrum of theenergies of the emitted particles. A characteristic feature or the

spontaneous beta disintegration of a nucleus is the continuous dis-tribution in energy of the emitted electrons from zero up to adefinite maximum energy. Thus, it is reasonable to adopt Pauli'ssuggestion that another particle with zero mass and zero charge,must escape simultaneously with the beta particle, carrying off avariable fraction of the available energy. This particle is termedneutrino and it was proved by experiments that it really exists.Some beta decays may first produce an excited state of productnucleus followed by the emission of a gamma ray in order to get toa ground state of energy.

Fermi suggested that both e and e+

(see below) and neutrinoare actually created at the moment of decay.

c. Gamma Emission. The gamma rays produced from a radioac-tive product give a line spectrum corresponding to a number of dis-crete frequencies in the emitted radiation. Gamma radiation origi-nates in the atomic nucleus, while x-radiation originates from theorbital electrons. It should be noted that gamma rays are emittedby the daughter nucleus and not the parent nucleus. Usually theyare emitted immediately after the alpha and beta particles. However,when there is a delayed gamma ray transition from an excited stateto a lower energy state, it is called an isomexic transitkon andis regarded as a pure gamma emission, e.g. Br ..4 Br".

d. Positron Emission. The positron is the positive electron(e

+) whose rest mass is the same as that of the electron and whose

charge has the same magnitude as that of the electron but the op-posite sign. This type of disintegration occurs where there is adeficiency of neutrons. Positron emission results in a decreaseby one unit in atomic number.

7"1,1

6V13 ^13 e+

As an alternative to emitting a positron, some nuclei capturea negative electron from one of tte atomic electron orbits nearestto the nucleus. The electrons which are most likely to do this arethose nearest to the nucleus i.e., K shel:; less often L-electronsare captured e.g.

Cu64

+ e NI611 + v(neutrino)

There is no nuclear radiation except for the unobservableneutrino from a pure K or L capture process. The secondary processis that an electron from an outer shell jumps to fill the vacancyin K or L shall immediately with the emission of a K or L x-raycharacteristic of the daughter nucleus. This process is calledElectron Capture and occurs when 'he amount of energy availablein a nucleus is less than 1.02 MeV.

e, Nuclear Fission. A heavy nucleus is broken down intotwo medium heavy fragments with a..out the same mass. The fissionof a nucleus is accompanied by a release of energy, since thebinding energies of the product nuclei are greater. The most im-portant difference between fission decay and the other types of

- 5 -

decay is that the masses and energies of the products of fissionare all statistically distributed.

Nucleons are bound together in a nucleus by nuclear force andthis force tends to keep the nucleus in spherical shape (the moststable configuration for a heavy nucleus). At the same time theCoulomb force between protons tends to make the nucleus unstableand this force is sufficient, in the heaviest nuclei, to upset thestability of the nucleus. It was found that this process happensto the nuclei whose atomic numbers are near or over 100. So itcan be seen that it is possible for spontaneous fission to occur,and it does indeed take place in a number of the heaviest nuclides.

Usually fission is induced by bombardment of nuclei withbombarding particles, preferably neutrons. For example

92U235

+0n1

92U236

ZxXAx

+Zyy

Ay+ 2.5

0n1 + energy

ZxXAx

andZv

YAy are fission products. The fission products

were found to have mass numbers from 70 to 160.

The important nuclides strontium-90 and cesium-137 are foundduring the process of nuclear fission. The energy liberated inthis process is just equivalent to the loss of mass. One unit ofmass on the C12 atomic weight scale is equal to 1.66032 x 10-24 gm,which, from Einstein's equation, corresponds to 931.437 MeV.

The main modes of disintegration are summarised in Table II.

Table II

charge mass Resultantnucleus

Example

Alpha particle ( a),nucleus of He (rangein air = 0.5 cm for1 MeV particle)

Beta particle ( 13), .

electron originatedfrom nucleus (rangein air can be as muchas several metres)

Gamma ( y), electro-magnetic wave (verypenetrating)

+2

-1

0

4

0

0

Z decreasesby 2, A de-creases by 4

Z increasesby 1, Aremains con-stant

change in

erier"level5131.--->"5only

2u238--v.90Th234

23 4 234PaTh ---e0 $ 91

80mBr

803

- 6 -

1.3 Radioactive Decay. The disintegration of radioactive nucleiis a purely random process. The decay of radioactive isotope isa first order reaction, i.e. the decrease per unit time in thenumber of atoms of a radioactive element, due to disintegration,is prcportional to the number of atoms which have not yet disinte-grated:

Number of decaying per second = a proportionality constantcharacteristic of the iso-tope times the number ofnuclei remaining

In the notation of the calculus, this is

-dN/dt =,AN (1)

Where N is the number of nuclei remaining, and Xis the pro-portionality constant for the isotope. It is called the decayconstant. If at some particular time there were Nonuclei in thesample, then we can find an expression for N, the number remainingat any later time t, by integration of equation (1). The resultcan be expressed in various equivalent mathematical forms:

N = N0 e-Xt

(2)

in(N /N0) = - At (3)

log10N 1°g10N0- 0' 4346xt

At any instant of its life, the nucleus has a definite pro-bability, unaffected by its age, of disintegrating during the nextsecond. The higher this probability, the more rapidly the nuclidemay be expected to disintegrate. If there is a 1% probability ofdisintegration in the next second, it is not possible to statewhich particular nuclei will undergo this process, but one can onlysay that 1% out of a very large number of such nuclei will decayduring this time interval.

It is not possible to speak of the total life of a number ofatoms. A useful constant is the "half life", the time needed forhalf of the radioactive atoms to disintegrate. After one halflife (i.e. after t= tk and N /N0 = 4) equation (3) becomes

In - At,

or 2.303 log4 = - Xt1/2

ti0.693

=A

It can be seen that equation (4) is the equation of a straightline. The half-life can be found from the plot of log N against t.If the plot is not linear, there is more thah one radioactivenuclide present. The half-life for any particular nuclide is acharacteristic constant. The determination of half-life is often

- 7

used for characterising a particular nuclide. The values of half-life vary, from one nuclide to another, from a small f::%:t.;r1 oftime to many millions of years.

1.4 Activity

An activity of 1 curie simply means that 3.7 x 1010

disinte-grations occur each second, irrespective of the radioactie nuclideconcerned. More convergent units are the millicurie (1r,- :14fi?:.;

and the microcurie (10-° curies). It is important to realise thaf,the activity of a sample refers to the number of disintegrationsper second, and not to the count rate recorded with a particularpiece of equipment. Count rate depends on the overall efficiencyof the counting system and the number of radiations emitted perdisintegration (e.g. sometimes two or more gamma ray photons areemitted in each disintegration).

1.5 Radioactive Growth

If radioactive nuclide "A" decays into a nuclide "B" whichis also radioactive and this is in turn disintegrate to "C" whichis stable we have,

AA

).

B*A*

---> B ---> C

When the rate of formation of B* is equal to the rate of dis-integration of B to C (or the rate of formation of "C"), the stateis called equilibrium.

There are three special cases which are worthy of interest.

a. Secular Equilibrium. Here the parent A has a much longerhalf life than B. It can be shown that the activity of B becomesapproximately constant after a time equal to 5 half-lives of B andthe total activity of the preparation of A and B any time is thesum of the initial activity of daughter at that time, For examplein the series

92rs

U238

90

091P a

241t, 4.5x109y 24.5 days1 2

49oTh

230 < a

t, 2x10 6 yrs

Olt 1.14 min.

9U234

Ali tbe members of this series have shorter half life thanthat of U23°, therefore in the uranium compound or uranium ore leftunprocessed for long time so that equilibrium has had time to beestablished, all,Asmbers of'the series are in radioactive equili-brium with the U" and with each other. This radioactive equili-brium presents a considerable practical application for example 11:fa source of relatively short lived radioactive tracers (like Pa234)for chemiea), biochemical and physical application by process of"milking" them as required.

b. Transient Equilibrium. The parent is longer lived thanthe daughter but sufficiently small for decay to be noticeablewithin the time of observation. In this case the activity of B*decreases with the characteristic half life of An For example

Ba140 0

'y

lo La1140 Ce140 (stable)

tk 12.8d ti 40 hrs.?'

c. There is no equilibrium when the parent has a shorterhalf life than the daughter (AA >AB ). If the parent is separatedfrom its daughter in the beginning, as the parent decays the num-ber of daughter atoms will rise, pass through a maximum value andafter the parent has decayed, the daughter is no longer formed,and will decay with its own half-life. For example

, 143 0 nr143---->

t, 33 hrs. t, 13.7d.

1.6 Tracer Techniques

Radioactive isotopes and their stable ones are essentiallychemically identical. In addition, the chemical reaction of theformer can be traced by means of their characteristic half-life ortheir disintegration product or the properties of the radiationthey emitted etc. When the radioactive materials are used insteadof their stable isotopes in any conventional chemical techniques,they are called "tracer techniques". Since the weight of radioiso-tope necessary to give a measurable activity is often less than10-15 grammes, i.e. unweighable amounts, it is therefore possibleby using these tracer techniques, to study the behaviour of mate-rial in micro, trace, or even ultramicro quantity either qualita-tively or quantitatively.

a. Tracers or radioactive indicators. These are radioactivenuclides used to follow the movement of the atoms of certain ele-ments or compounds throughout any mechanical, chemical or biologi-cal systems. They can be divided into two types.

(i) Isotopic tracers. Radioactive tracers and their stableisotopes are chemically identical. The tracer and the stable iso-tope must have the same chemical form so that they can act in thesame manner throughout the process. In the system involved withelements of several valency states, it is advisable to make surethat they are in the same form by repeating oxidation-reductioncycles several times. lootopic tracers are used to trace elementsor compounds in question from the beginning to the end of the pro-cess, such as in studies of metabolism in the body, diffusion, rateof reaction, analysis etc.

(ii) Non-isotopic trace's or physical labels. These areused when the experiment is not concerned with any particularelement or atom. The tracers need not be identical with the stableelement. For example, radon gas has been mixed with air to studythe flow patterns of air jetted into a furnace to improve combus-tion. Non-isotopic tracers may be used for locating a solid or

- 9 -

liquid object. They have been used to discover leaks in compli-cated systems of pipes and tubes.

b. Carriers. These are the stable materials added to theradioactive samples and they act as carriers for the active mate-rials in all subsequent reactions.

From the decay equation, -dN/dt =AN, the weight per curie ofnuclides of different half-lives may be calculated. In many casesonly 10-.3 microcuries can be detected and measured. Hence, inradiochemi9a1 work, the masses involved are very small (of theorder 10-1° to 10-19 grammes) so small so that solubility productscannot normally be exceeded and precipitation is, therefore, impos-sible. In order to carry out precipitation and reduce effects ofadsorption it is necessary to use "carriers". A sample containingno carrier is called "carrier free". There are three types ofcarriers.

(i) Isotopic carriers, Carriers and radioactive nuclideshave the same chemical form and state. This type of carrier isused when high specific activities (disintegrations per unit timeper gramme) are not necessary.

(ii) Non-isotopic carriers. A chemically different elementis used to carry the radioactive tracer. This type of carrier isused when no isotopic carrier is available or when the sample ofhigh specific activity is needed because this carrier may be sepa-rated from an active material at the end of the experiment and pureactive material in the carrier free form is obtained.

(iii) Scavenger type. Where no other suitable carrier mayexist, adsorption on the surface of a precipitate such as ferrichydroxide, or almost any gelatinous or finely divided precipitate,cAn be used to carry out precipitation of certain radionuclides.

Most radioactive isotopes for use in medical, agriculturaland industrial research are made in the nuclear reactor. Thereactor can be regarded as the machine that produces neutrons andan enormous amount of energy as a consequence of the controlledfission process (see fission on page 4 ). The reactor furnishesa bombardment of neutrons that attach themselves to the nucleior centres of the atoms of an ordinary element that is called the"target material," The typical examples of the production of radio-isotopes by the neutron bombardments of the target matA-=3.als Inthe nuclear reactor are given below.

,32 +ip

1or

32032 4.. n r "32 (n,9) 15P16° 0 ;6°

11Na23 +

0n1---->

11Na24 + y or 11Na24

3LI

60

+ n 03

4. 21eor ,L16 {n ,a) 0 3

10

1.7 Methods of Detections of Radiations

A very important effect of emission of radiations is to pro-duce ionisation in air and other materials and this ionising effectis the basis of the detection systems. Beta particles and alphaparticles produce ionisation directly; gamma rays by the chargedsecondaries which they release and which are directly measurable.A gold leaf electroscope can be used as a simple detector for thepresence of ionising radiations (see later).

Beta particles, alpha particles and gamma rays may be detectedby the use of some nuclear detectors which make use of the ab.11:tyof radiations to ionise air or other gases such as an ionisationchamber, a proportional counter (suitable for the counting of analpha or a beta particles with low energy), a Geiger Muller counter(suitable for the detection of strong beta emitters), a scintilla-tion counter (suitable for the detection of gamma rays or weakbeta emitters, the design of this counter based on the ability ofelectrons to cause fluorescence in certain substances) in conjunc-tiun with an electroscope, a scaling unit or a rate meter.

The ability of the radiations to darken a photographic plateor a film is found useful in the technique called "autoradiography"(see later).

1.8 Operational Problems in Tracer Experiments

According to the report of I.A.E.A./UNESCO Panel of Expertson Nuclear Science Teaching, BangkokoJuly 1968, three levels ofradiation hazard may be distinguished corresponding to the dif-ferent educational levels.

Less than 16 years. This.may be regarded as the level at whichradiation doses are trivial and hardly discernable from dailybackground. Most schools with science laboratories will be inthis classby virtue of chemicals in store or television sets usedin teaching. The radiation levels for students experiments shouldnot exceed those recommended in 5.1, 1.3 (ii), (iii), (iv), (v)of the I.A.E.A. Safety series No 9,1967.

16-18 years. Student experiments with unsealed sources in thisage group (and teacher demonstrations to the under 16 group),should be such that activities do not exceed those listed in column7 of Table II A and B of the I.A.E.A. booklet Safety Series No.9,1967. Experiments with sealed sources may use higher levels ofactivities, for example those specified in the booklet issued inthe United Kingdom by the Department of Education and Science(document AM 1/65).

It may be desirable to place an upper limit on the totalamount of radioactive sources kept in store in any laboratory.Waste disposal is not a problem at these levels of activity andunsealed sources may be disposed of as for ordinary chemical waste,

Above 18 years. In this group, universities and colleges will be

covered either by the levels referred to above, or by licences forhigher activity levels issued by national authorities.

Radioactive sources of the type and activities used in thishandbook require only normal, good chemical practice for theirea.e handling. Uranium, thorium and potassium salts are verypopular sources of radiations since they are quite safe to workwith (normally used as a chemical agent) presenting no radiationhazards. Besides, uranium and thorium salts can be used as a"cow" so that many isotopes can be milked off. Uranium oxide isused regularly as a reference counting source due to the long halflife of uranium-238. Monazite sand can also be used as a countingsource. The general guide lines for physical considerations aregiven below.

The experimenter has to consider:

a. Health hazards and contamination control. This needspractice and discipline. Radioisotopes are useful because of theradiations they emit, but at the same time they present a potentialhazard to the isotope workers and, in certain circumstances, tothe general public. All of us have learned to live with fire andelectricity which are dangerous as well as useful, we can thereforelearn to live with radiation too. Danger from radiation dependson the degree of exposure. The exposure to radiation can be li-mited in three ways:

(i) Distance. In general, the effect of radiation falls offwith distance; the inverse square law can be applied.

(ii) Shielding.

(iii)Time. Stay in the vicinity as short a time as possible.

b. Nature and energy of the emitted radiation. This is im-portant in relation to the method of detection of the radiation.

c. Half-life, A suitable half-life nuclide is to be chosenfor a certian experiment, that is a nuclide of sufficiently longhalf-life to maintain its activity up to the end of observations,but not so long that disposal problems arise.

d. Chemical procedure. This has to be carefully chosen tosuit the properties mentioned in b. and c. of this section.

e: Isotope effect. The chemical identity between isotopesor the some element is not complete among the light element, e.g.H and 11'; the mass differences between these isotopes are greatenough to cause different chemical properties. The effect, owingto the large difference in mass, is pronounced in any reactionwhere mass is involved. Thus, there are changes in rate of reac-tion, equilibrium constant or bond strength etc. For all elementswith atomic mass above carbon this isotope effect is small tend canusually be ignored.

f. Dummy run with non-active isotope is always essential.

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Most, if not all, of the difficulties concerning manipulations willbe solved as known beforehand, hence contaminations and healthhazard are minimised.

Radioactive materials should only be used when there is noother equally suitable method available. There are numerous usesof radioisotopes, where the hazard from emitted radiations is great,but can be controlled. Precautions, through understanding of thesubject and training will minimize this hazard considerably sinceto know is to understand and to understand is to act with wisdom.

- 13 -

References

1. DHABANANDANA S. Dissertation for Post Graduate Diplomain Radiochemistry, Leicester College ofTechnology and Commerce, 1960.

2. FAIRES R.A. and "Radioisotope Laboratory Techniques" ,

PARKS B.H. 2nd edition, George Nevnes Ltd.London 1960.

3. HARVEY B.G. "Nuclear Chemistry",Prentice-Hall, Inc.,Englewood Cliffs,N.J. 1965.

4. CHOPPIN GREGORY "Nuclei and Radioactivity",Benjamin Inc.Nev York Amsterdam,1964.

5. CARSWELL D.J. "An Introduction to Nuclear Science,et.al. "The Australian Atomic Energy Commis-

sion,1965

6. IAEA/UNESCO Panel of Experts on Nuclear Science Teaching(report), Bangkok, July 1968.

7. Document AM 1/65, Department of Education and Science, U.K.

8. IAEA Safety Series, No. 9, 1967.

9. CARSWELL D.J. Introduction to Nuclear Chemistry,Elsevier Publishing Company, Amsterdam -London - New York,1967.

PART II

EXPERIMENTAL SECTION

- 15 -

A. COUNTERS, STATISTICS OF COUNTING AND SOURCE PREPARATION

When a radiochemical work is carried cut, it is almost alwaysthat the experimenter has to deal with counting by at least oneform of counters, Thus it is essential to know about the perform-ance of the counter including counting and the preparation of acounting source.

A.1 Counters

Only a simple Geiger counter with an end window Gieger tubewill be discussed here. A circuit diagram of the type used by theauthor is shown. The G.M. counter is operated at a potential de-termined by its characteristic curve. The curve 1s obtained byplotting the count rate due to a fixed and constant radioactivesource as a function of voltage across the counter. At a low volt-age (before A) the pulses are too small to operate the counting

Countrate(c.p.m.)

Voltage across the counter

Fig. 1 Characteristic curve of a counter.

equipment 'he "starting voltage" A is reached where the pulses be-come large enough to trigger the equipment; and there follows a"plateau" where there is little change in the count rate as thevoltage increases over the range say 100-250 volts. Beyond thisthe count rate shows a very steep rise and the tube may go intocontinuous discharge. In order to ensure stable operation thecounter is worked with an applied voltage roughly corresponding tothe centre of the plateau. The first task of the user of any typeof counters is to determine its characteristic curve by takingreadings from a radioactive source at small voltage intervals untilpoint C in the graph above is reached, results are plotted and theoperating voltage is obtained. This should be carried out regular-ly. However, most of the school type counters are produced in sucha way that the users will encounter with a minimum complication

- 16 -

such as the G.M. tube used with such a counter oile receive s. fixedvoltage supply correspond to the operating voltage of that tube,hence there is no need to determine an operating voltage:

A,2 Statistics of Counting

Radioactive atoms decay statistically, that es the time atwhich one atom will decay is independent of the decay of al) otheratoms and cannot be Iceditea, This randomness of the decay ofradioactive atoms can and should be illustrated in the class be-cause its consequence concerning errors in counting of activity isvery importeat in radiochemical work. Two suggestions for theillustrations of this property are given below.

1. It can be shown by an analogue experiment such as thefamous one; throwing coins. Student throws 100 coins or more col-lects and counts only those turn tails up, discards those with theopposite faces up or vice versa. Toss the remainder, collect andcount those with tails up again and discard the rest. This processshould continue as many times as possible. It can be shown fromthe results that the most probable event is that half of the totalcoins will turn heads up and the other half turn tails up and theleast probable one is that every coin turns only head or tail up.This is applied with a large number of coins and a large number ofevents. The plot of the logarithm of number of coins at the endof each event against the appropriate eventi,e., first event,second event and so on should give a straight line with a negativeslope and this experiment can be used to explain the meaning ofexponential decay and half-life.

2. Fix source at a position under a G.M. tube and repeat thecounting 50 times or moee. Record each count in counts per unittime. Results show that the coincidence of the values of the countrate is really a very rare event. In fact what normally happensis that the number of counts "N" obtained changed statisticallyabout the mean value "N". Therefore students should be waened thatthey should not expect the count rate will be exactly the same whenthey repeat the measurements even under the identical condition andthat the randomness of disintegration is responsible for this.

If count rates obtained in the above paragraph are arrangedinto group with a small intervals of count rate within each groupsuch as 25 or 50 and are plotted on the abscissa and the numbersof count rate that fall into each group are on the ordinate. Underthe perfect condition a bell shape curve should be obtained fromwhich a mean count, standard deviations may be read off as illus-trated in the graph below.

From statistical consideration, the c, of a 1:etof measurements is approximately equal to the square root of themean count (or cf the count rate itself in the case of a singlemeasurement). It imaediately follows thEvi

POOR ORIGINAL COPY - BESTAVAILABLE AT TIME FIUABQ

- 17 -

Total counts Standard deviation(o) Limits of counts

100 47515 1001.10 (10% error)

1,000 h71756 1,000+31.8 (3.2% error)

10,000 415-766 10,000+100 (1% error)

Frequency

GroUps ofcountrates

-0 +a

Fig. 2 Statistical distribution of the count rate

It is advisable therefore to collect as many counts as possi-ble in order to reduce the statistical error to a minimum. It isa common practice that the counting time should be long enough toobtain about 10,000 counts. However, it may not be practical inthe counting of a very low activity source since one has to waittoo long in order to obtain a total count of that order. In sucha case it is left to tt : experimenter to decide about the lengthof time that can be spent in the measurement of the activity, bear-ing in mind that the lower the total count the higher is the errorof the counting.

4

- 18 -

A.3 Source Preparation

A reference source, that is a source with a constant activityover 7:any many years due to its long half life, is usually madefrom uranium compounds. For example, uranium oxide is made aslurry with a little acetone and "Durofix" or "Duco Cement" or thelikes and is spread as a thin film over the bottom of a small alu-minium or stainless steel source tray about 1 inch in diameter andis dried under an I.R. lamp so that the boiling of the sample isavoided. A thin foil of aluminium is placed on top of the trayand the source is counted at 2 cm below the tube window, about10,000 - 15,000 counts per minute(c.p.m.) are aimed at. If thefirst layer does not yield high enough count rate, following layershould be applied successively until the desired count rate isreached. A little of "Durofix" or "Duce Cement" is then smearedaround the rim of the tray and the aluminium is stuck onto it.

If monozite sand is used as a source of radiations, the abovepr:edure may not be suitable since the activity from the sand isusually not high enough. Instead, A small amount of the sand isput on a small watch glass and the ;lass is placed immediatelyunder the tube and is counted.

A point soureE mn be made from a solution of the radioiso-tope concerned. A hypodermic syringe is used to transfer thetracer solution from its container, usually a nultidose vial and asmall drop is placed at the centre of an aluminium or a stainlessster1 source tray or a planchette as it is normally called. Thesource is dried under an I.R. lamp and is checked for its countrate. more drops can be added if a higher count rate is required.Finally the source is sealed with a piece of :ellotape.

However, if one wants to spread the radioactive solutionevenly over a larger area one can cover the bottom of the tray witha piece of a filter paper circularly cut to fit exactly that partof the tray, and a few drops of A radioactive solution is appliedwhich will spread evenly over that piece of paper and the frayedis covered in the usual manner.

It should be stressed that when making up a counting sourcefrom a radioactive substances, surgical gloves should be worn andprecautions should be taken as to avoid contamination of the ax-perinetter and the working area. The dispensing of radioisotopesshould be tarried uut on N mall tray which is covered with a pieceof eo -lbsorbent paper.

POOR ORIGINAL COPY- BEStAVAILABLE AT TIME Fawn

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B. PROPERTIES OF RADIATIONS

Radioactive elements undergo spontaneous transformation fromOne chemical atom into another; this is accompanied by emission ofradiations whose basic properties are mentioned on page 3-5 of PartI. The following experiments illustrate various properties ofradiations both with and without the aid of a nuclear detector.

Xxveriment B.1 Deflection of Radiations by Magnetic F.

Alpha, bets and gamma rays are characterised by their be-haviours in the electric and magnetic fields. The rays bendingtowards the left in the diagram carry positive charges and arecalled alpha (a) rays. Those deflected to the right carry negativecharges are called beta .S) rays. The third type gamma (y) raysgo unaffected as they carry no charge at all. The deflection ofthese rays by the magnet can be shown quite conveniently in theClass.

Fig. 3 Deflection of radiations by magnetic field

Materials Required

Alpha sourceBeta source point source should be used.Demme sourceHorseshoe magnetCollimatorDetector

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Procedure

A beam of radiation to be tested is collimated through asmall hole so that the scatterirg of the beam is minimised and themajority of the radiation emitted from the source- will be efficient-ly detected. The count rate is recorded after which the magnet isplaced on top of the collimator (This can be made of any lightmaterial in the form of a block with a small hole, of the same sizeas the source, in the middle through which radiation pases). Ifthe source is a gamma one there will be no change in the count ratebut if it is an alpha or beta source, the count rate will be re-duced considerably, the remaining count rate is due to backgroundradiation and to the gamma component which always contaminatesalphaand beta sources to some extent. .Scanning of the G.M.-tube insearching for the deflected radiations reveals that they are de-flected as indicated in Fig. 3, and that beta particles are de-flected to higher legree than alpha particles owing to its low mass.

Experiment B.2 lonisin PIJ32o2eLx.t of Radiation

When a charged particle, passes through matter, it willeither excite or ionise the atom; an orbital electron is raised toa state of higher energy within the atom in the former case andejected in the latter. To demonstrate the effects caused by airionising materials e.g. flames, glowing wood splinters, hot wire,a-,e-rays, a "home made" electroscope can be used. It is easy toconstruct by students in the class.

Materials Required

A flask or wide neck bottle with fitted rubber stopperGold foilA thick copper wire or a big screw nailA copper plate (e.g. door knob)Plastic comb, ruler or fountain penAn alpha or beta source.

Procedure

The electroscope consists of a conical flask, into which isfitted a rubber stopper. The stopper carries a thick copper wireor a long screw nail. A big piece of a copper plate such as adoor knob is soldered to one end of this wire. The other end ofthe copper wire (inside the flask) is flattened, and two strips ofa gold foil about 1/4 inch wide and 1/2 inch long are stuck on toit. The electroscope is charged with a plastic comb, a ruler ora fountain pen which has been rubbed with a piece of wool or satin.The a-particle source is brought near but not t-:'uching. The leavesshould slowly come towards each other, shoving the a-particles fromthe source are ionising the air and thus making it a conductor.This can be compared roughly with the effect of heat such as from alighted match Or sunlight.

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Ionisation can also render the grain of a photographic emul-sion developable. When a photographic film is exposed to ionisingradiations, it is blackened to an extent depending on the amountof energy absorbed. The method of detection of radiations makesuse of this effect such as in the device called "film badge" whichis used it the measuring of the integrated dcse that was receivedduring a previous period. Use is also made of this effect ofradiation in the technique called "autoradiograph". An example ofthe application of this technique is given in experiment C.1,2.

Experiment B.3 _Absorption

When charged particles (i.e. a-, 6-particles and secondaryelectrons resulting from interaction of y-raye with matter) passthrough matter, they will lose energy through the ionisation theyproduce. The more they produce ion-pairs the lees energy left andthus the less fav they can travel. The distance travelled by acharged particle in matter is called "range". In a particularmaterial the energy loss is proportional to the mass per2unit areaof the absorber e.g. mg/cm2 (range in aluminium) or g /cm (rangein lead). Among the three, a-particles have the lowest penetratingpower whereas 0-particles have a medium one and y-rays are the mostpenetrating of all.

Equal thickness of a given material absorbs the same fractionof the incident y-radiation i.e., the absorption is exponential anda plot of the logarithm of counts per minute against thickness ofthe abskrber is linear. The validity of the inverse square law(la 1/(14, where I is the intensity of light or radiations and d isthe distance) exists in the absorption of y-rays, but this lawdoes not hold in the 8- radiation absorption. The attenuation of8- radiation depends on the density of the material and in mostcases is independent of atomic weight. The natural 6- spectrum hasa definite maximum energy (characteristic of each radionuclide)and, corresponding to this, a maximum range. If a series of anabsorbing material are placed between a 8-ray source and a detector,it is found that activity of the source detected decreases as thethickness of the absorber is increased up to a limit where no sig-nificant activity is recorded at all, While the absorption of B-and r -rays is quite simple to observe using an ordinary O.M. counterand a set of different thicAnesses of the absorber (usually alumi-nium or lead absorber is employed), the absorption of a-radiationis not at all easy to study with ordinary counter owing to the veryshort range in matter of a-narticles, about half of the alpha energyis used in penetrating the reiger tube window,

Materials Required

O.M. counterAlpha sourceBeta source (such as 02)Oamma source (such as Cs1-57)Get of aluminium absorbers with different thicknessesSet of lead absorbers with different thicknesses

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Procedure

A beta source is placed under the G.M. tube and the countrate is recorded. A series of an aluminium absorber with differentthicknesses is placed between the detector and the source succes-sively and the count rate is recorded for each one. The count rateshould decrease little with a thin foil of aluminium but more whenthe thickness is increased until there is no appreciable change incount rate with increasing of the absorbers thickness, indicatingthe 0-particles of all energies up to the maximum emitted from thesource are absorbed. The minimum absorber thickness required tostop all particles from entering the G.M. tube or any nuclear de-tector is termed "maximum range" and can be obtained accuratelyfrom the graph of log. c.p.m. against thickness of absorber (rig.4)

log c.p.m

Absorber thickness

maximum range

Fig.4 Absorption curve for 0-particles

The maximum energy is calculated from the relationship.

En, 0.00185 R 0.245 (R is the range in mg/cm2

and isgreater-tfian 300, E is the maximum energy in Mev). Alternative-ly, the maximum 8-energy gy may be found from the standard curve ofmaximum 0-energy against maximum range in aluminium.

When the same procedure is employed for the determination ofthe y-ray absorption using lead absorbers, the plot of log c.p.m.vs absorber thickness is linear. Hence there is no maximum range,instead, a half thickness is used and it is a characteristic of aparticular nuclide, The half thickness is the thickness of lead

4

-23 -

absorber needed to reduce the count rate to one half, The corres-ponding energy of a can be read off from the standard curveof half thicknesses against energy.

However, for the pre-university level, is quite sufficientto carry out experiments tiC: far as to show the existence cf themaximum range for a 0-emitter and a half-thickness for a y-emitterand that those values are different for different isotopes.

It is also thought worthwhile to compare the penetratingpower of B-and 1-radiations, If the position of the GM-- tube isfixed and the position of the source of radiation is = i withrespect to that of the tube (1-2 mm at a time). It irt found thatthe order of the distance travelled, in Air is u<B<y.

The inverse square law can be studied for B- and y-radiations.This time, the positions of the source with respect to that of thetube are known for each count rate,,recorded. For r-emitter, a plotof count rate against 1/(distance)`; or even better a plot of

square root of reeiprocal of count rate against distance yields astraight line hence the inverse square law is valid. This is nottrue for a 0-source. It is appropriate therefore to point out tothe students that particles of different energies are absorbed bydifferent distances in air. The deviation from the inverse squarelaw for 13-radiations is attributed to the fact that this extraeffect is added on, otherwise the simple inverse square law ofintensity of radiation would be valid.

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C. SEPARATION TECHNIQUES

It is quite important for radioisotope users to be familiarwith the normal methods employed in the separation of radioisotopes.The recommended isotopes for use in schools are either short livednuclides or nuclides belonging to the decay chain of uranium andthorium. If the latter type of nuclides arm used, one neeas toseparate the desired nuclide from other members of the chain. Theradiochemical purity thus can be checked by the type and energy ofthe emitted radiations and/or by means of its characteristic half-life. Methods which have been used for clean and efficient separa-tion of radionuclides are as follows: volatilisation, distilla-tion, electrodeposition, precipitation, solvent extraction, chroma-tography, and ion exchange. The experiments based on some of theabove techniques are described below:

C.1 Paper Chromatography

Paper chromatography is also employed in the separation ofions in radiochemical technique as well as in the ordinary analy-sis. The principle is that when a suitable solvent flows by capil-lary action along a strip of filter paper, any spots of materialwhich may be on the paper, and which are soluble in the solventused will travel along the paper in the direction of the solventflow. The distance travelled is dependent on the partition coeffi-cient (the various components of the original mixture move with thesolvent but at different rates). It is customary to calculate theRf-factor for each separated nuclide. This is defined as

distance travelled by the spot frontR =f distance travelled by solvent front

Rf -factor of an ion has a characteristic value under the

given conditions.

As far as the activity is concerned the microcurie quantityis usually sufficient to be detected radiochemically. The actualnumber of radioactive atoms present however is so vanishinglysmall that it will be adsorbed on the paper support and on the wallsof the apparatus. To eliminate this, small quantities of the iso-topic carrier are usually added.

Experiment C.1.1 Radiochemical Purity of 32POL3 by AscendingPaper Chromatography,

Material Required

as jar lid + paper holderStrip of chromatograRhy taper (preferably Whatman No.1)strong carrier free 32P0 solutionA 4% vol/vol nitric acid in methanolfarrier solution (10% vol /vol ortrophosphoric acid solu-tion)"pray solution (5 g ammonium molybdate dissolved in100 ml water poured into 24 al nitric atid and 2h alwater with stirring.

- 25 -

Procedure

1. A strip of paper 30 cm long x 3 cm wide is marked out inpencil as follows. A line is drawn across the paper 2 cm from oneend and this is called a base line. The paper is now marked outin cm from this base line and numbered.

2. A 4% vol/vol nitric acid in methanol is poured into agas jar to a depth of 2 cm. The length of the paper strip is thenadjusted (outside the jar) so that the base line will be about 1 cmabove the liquid level when the apparatus is assembled. The papershould not be wetted at this stage.

3-3. A drop of carrier free 32PO4 solution is now appliedin the middle of the base line (the minimum of about 5000 countsper minute under a thick window 0.t4. -tube should be aimed at) anddried under an I.R.-lamp, after which a spot of a carrier solutionis added in the same position and again is dried.

4. The apparatus is now assembled using a jar lid and paperclips or a piece of "cellotape" to fix one end of the paper on tothe lid (Fig. 5). The paper must not touch either sides of thegas jar and is allowed to develop for 2 hours. The position ofthe alcohol front is marked on the chromatogram.

Pig. 5 Paper chromatographic set-up

- 26 -

5. The strip is dried under the lamp and sprayed with asolution of ammonium molybdate to give a yellow spot for carrier.The strip is then dried again and is cut into 1 cn pieces as faras the alcohol front and each piece is counted using a 0.M.-counter.The counting rates are plotted in the form of a histogram. FindRf-factor. Alternatively, the rioactive spot may be located by

autoradiography (see later).

If time of observation is available this expertperq can beextended to illustrate the separation of 32Fq- from "S0t-. Thisproved a useful method since S35 is the most probable contaminatingnialide in the preparation of P32 by the production process:0.4(n,p) P32.

Unfortunately, the R4.-factors of PO4 - and SO4- are veryclose, hence the time needed for the separation by paper chromato-graphic method is about 22 hours. A 35 cm long x 3 cm wide paperis used whilethesolvent is 7.5% vol/vol nitric acid in butanol.

Experiment C.1.2 Separation of Uranium Decay Chain Isotopes

The beginning of the uranium disintegration series is asfollows:

U238 a 234

> U-X1 214 d. 11-X2 (Pa )

4.5 x 10 9 yrs.(Th234)

/0 1.1 min

Io

a U-II (U234)(Th230) 2.3xt0- yr&

If a uraniim compound has gpt. been chemically treated forseveral months U3° and U-X

1(Th

2'"

) will be in secular equilibrium(see page 7) and may be separated by suitable methods one of whichis ascending paper chromatographic method.

Owing to the low energy of the 0-particles emitted by U-X1,this nuclide is counted by allowing its daurhter product U-X0 tocome to secular equilibrium and counting the energetic 0-particlefrom this. Fortunately, the half-life of U-X0 is short (approxi-mately 1 minute) and the equilibrium is quickly established within10 minutes.

Material Required

Apparatus - the same as in experiment C.1.1Saturated uranyl nitrate solutionHexane or butanolAct. one

5% potassiu. l'errozycnlat sclation

- 27 -

Procedure

The general procedure is similar to that of experimentC.1.1. Enough uranium nitrate solution is applied on to the originof the chromatogram (500-600 counts per minute total) dropwise(dried after the application of the each drop). The chromatogramis then allowed to develop for 3 hours in 1:1 hexane-acetone mix-ture or about 2 hours in 1:1 n-butanol-acetone mixture. The chro-matogram is dried and sprayed with 5% potassium ferrocyanide solu-tion, a dark brown spot appears near the solvent front due to U 238 .

Usigg a 0.M.-counter the low penetrating a-particles emitted byU23° are not detected, whereas radio chromatogram scanning detectsthe thorium spot near the base line. Since half-life of Th234 is24 days, this can be checked by scanning the strip 24 days later,the thorium activity will decrease by half of the original value.

It is obviously seen from the experiment C.1.1 that a longerstrip of paper and a longer time allowed for the ascending of thesolvent facilitate the clear separation of the phosphate and thesulphate and this is also true for this separation. The hexane-acetone system provides a cleaner separation than the n-butanol-acetone system. On the other hands, the latter one has the advan-tages of being a cheaper and a faster separation system.

gxperiment C.1.3Autoradiography

In the preceeding experiments the Ions under study are col-ourless and hence producing "invisible chromatograms" . Withthe aid of suitable developing agents such as ammonium molybdate .

and potassium ferrocyanide solutions they can be made visible. In

addition, if the ions are labelled with radioactive isotopes, itis possible to show up their positions by counting. Alternatively,use may be male of the power of the radiation to blacken a photo-graphic emulsion; under a suitable conditions this will indicatethe exact distribution of the radioactive material although itwill not identify the nuclides concerned.

Material Required

X-ray FilmDeveloperFixerDark room

procedure,

Steps 1 to 4 in experiment C,1.1 are repeated. The chroma-togram is dried and a drop of active phosphate soluticn is addedon the origin and is dried (this to ascertain that the base linewould show on the developed film). 'The chromatogram is now placedbetween 2 pieces of cellophane and laid on a strip of x-ray filmand held securely by weights in complete dqxknesa for an appro-priate time (the usual exposure time for PJ4 is 2-3 hour andlonger time is needed for weak emitters such as 535 or CA"). On

- 28 -

subsequent development of the film, using conventional reagents*;regions corresponding to localisation of radioactive elements aredarkened so that photographic image of the tracer distribution isobtained. The experiment illustrate that radiations can blackenthe film emulsion.

A indoor plant such as "scindapsus aureus" growing in asolution of ammonium phosphate labelled with P32 for 24 hours(dilte solution of ammonium phosphate must be used to prevent osmo-sis taking place) is a fine object to be autoradiographed and theimage on the film provides the evidence that phosphate absorbedthrough roots. Alternatively, some letters can be written on apiece of filter paper using solutions of radioactive isotopes asink and autoradiographed.Any objects can be autoradiographed pro-viding ';hat they are radioactive and emit strong enough radiations.

Note that in paper chromatographic methods,separation canbe carried out using very minute quantity of the sample i.e. only1-2 drops are usually sufficient.

* Normal procedure is as follows:

1. Develop for 4-5 minutes at 25°C in developer2. Rinse in cold water (preferably distilled water)3. Fix for 15 minutes or longer in fixer4. Wash in running water for 1 hour or longer5. Hang up to dry

peveloper

nctr,

N.B.

Metol P3Sodium sulphite 150;Hydroquinone 8gSodium carbonate 100gPotassium bromide 5gWater up to 1 litre

Sodium thiosulphate 300gPotassium metabisulphite 25gChrome alum (chromium

potassium sulphate) 12.5gOlacial acetic acid 2.5m1Water up to 1 litre

1. Ready made developer and fixer are commercially availablewith the purchase of the x-ray film.

2. Developer and fixer are prepared according to what isgiven in "Radioisotope Laboratory Techniques" by FairesP.A. and Parks. B.H., Oeorge Newnes Limited, London (1960).

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C.2 Ion Exchange

Ion exchange methods are widely used in preparation andchemical analysis. Because of its great simplicity, the ion ex-change method is also becoming very common in radiochemical inves-tigation, not only as a method of separation but also for otherpurposes. It has been developed over a whole range of elementswith and without carriers. Basically, an ion exchanger may beconsidered similar to an insoluble salt, acid or base in which oneof the ions is freely mobile and the other fixed. The material istermed a cation (electropositive) or an anion (electronegative)exchanger according to the sign of the mobile ion. The mobile ionmay be exchanged, under suitable conditions for other ions of thesame polarity, when the exchanger is treated with a solution con-taining these ions. Equilibrium is established between the mobileions of the exchanger and the ions in the mixture. The retainedions are separated by washing repeatedly with a suitable eluent;those ions which are adsorbed least strongly moving down mostrapidly. In practice, a column technique is almost always used.

U-X1 (Th234

) may be extracted from uranium on a cation ex-change resfn. If a solution of uranyl nitrate is poured througha small column of strong cation exchange resin (such as Zeokarb-225), the U-X1 is more strongly adsorbed than the hexavalent ura-nium. The experiment below was developed at the Isctope School,A.E.R.E., England.

Experiment C.2.1 The Separation ofCarrierFreeU-)1Exchane

Materials Required

Cation exchange resin (Zeokarb-225 or similar)2.5M hydrochloric acid0.3M sulphuric acid12% uranyl nitrate solutionStand with clampsCylindrical separating funnel (capacity of about 10-20 ml) or

an ion exchange column with a jet, rubber connector andscrew clip

Petri dishG.M.-counter (coupled with a scaling unit or a rate meter)

procedure

An ion exchange column is prepared as follows:A small amount of cation exchange resin, stored in 2.5M hydrochloricacid or 1M sodium chloride solution (to ensure it is in H or Naform), is made into a slurry with distilled water and poured intothe. cylindrical separating funnel which is plugged with a fewglass beads and glass wool, The tap of the funnel should be closedduring this operation. The resin is allowed to settle and thewater run off until the level is about 2 cm, above the top of theresin. This is repeated, if necessary, until the column of resinis approximately 8-10 cm deep. Wash column with about 100 ml of

- 30 -

distilled water (the column must not be allowed to go dry duringthe whole procedure.

141. If an ion exchange column is used, the length of the columnmust be increased slightly in order to increase the separationefficiency and the use of a large column is advisable due to thelarge amount of ions present.

A 25 ml of 12% uranyl nitrate solution is run through acolumn of resin at about 2 drops/second into a Petri dish mountedat an angle so that the effluent liquid may be monitored by a G.M. -counter before running over the edge into a beaker (Fig. 6). Elu-tion with 100 ml of 0.3M sulphuric acid will remove most of theuranium but little of U-X The presence of uranayl ion in theeffluent can be checked using potassium ferrocyanide solution (redcolour). After a lapse of 5-6 months the radiochemical equilibriumbetween uranium and its daughters - U-X], U-X2 is reached and canbe used again as a source of U-X1 or U-X2.

To extract U-X0 (Pa234) from U-X1 (Th234 ), simply run 2.5Mhydrochloric acid through the column. The effluent is monitoredby a G.M.-counter and its activity will be seen to increase up toa maximum. At this point the flow of HC1 is stopped and readingsof the ratemeter are taken at 10-20 second intervals or 10 secondscounts are tfiken every 20 seconds using a scaling unit. The halflife of Pa234 is therefore determined either by observation or byplotting a decay curve if the decrease in count rate with restto time is recorded. Literature value of the half-life of Pa'."is 1.14 minutes.

Fig. 6 Ion exchange set-up

- 31 -

C.3 Solvent Extraction

The solvent extraction method is a powerful method of separa-tion and should prove extremely useful in the separation of tracerquantities particularly where speed of separation is required suchAs in rldiochemical work involving a short lived nuclide.

In the separation by solvent extraction, advantage is takenof the fact that, if a substance is soluble in two solvents whichare immiscible, it will distribute itself between the two phases.When equilibrium is reached the ratio of the concentration of thesubstance in the two phases is called the "distribution ratio" or"partition coefficient" which is a constant at a given temperatureand is independent of concentration. Usually in practice, the sub-stance in question is dissolved in a particular solvent, generallywater. A quantity of the immiscible extractant is added to thissolution, the two are shaken together, and then separated. Thismay need repetition, depending on the completeness of extractiondesired and the distribution constant. There are, however twoaspects of solvent extraction techniques which find wide applica-tion in radiochemistry. The first of these is the use of a speci-fic complexing agent in order to render the desired constituentsoluble in a particular solvent. The second method is by carefulcontrol of pH, with which it is possible to achieve rather completeseparations of the desired material from many other ions present.There are many types of apparatus used depending on the size of theoperation. The simplest one is the shaking of two phases togetherin a separating funnel (with pressure frequently and carefullyreleased by the removal of the stopper). The two phases are thenseparated by drawing off the bottom layer via the tap.

Since the cupferron-complex of protactinium was found to beextracted into benzene, chloroform, diethyl ether, amyl acetate,methyl isobutyl ketone, isobutyl ketone and nitromethane fromaqueous solution, ranging from pH 1 to 4 in acid, hence solventextraction of U-X2 (Pa23') from its parents was used satisfactorily.

Experiment C.3.1 The Extraction of Carrier Free U-X2by Solvent Extraction

Material required

Saturated solution of uranyl nitrate2% cupferron solution6M nitric acidAmyl acetateStand with clampG.M.-counter with a scaling unit or a rate meterMeasuring cylinder 10 mlSeparating funnel

- 32 -

Procedure*

,Take 10 ml of saturated uranyl nitrate solution (measuredwith a 10m1 cylinder) in a separating funnel. This is acidifiedwith 3 ml. concentrated nitric acid. Add 6 ml of 2% cupferronsolution in 10 ml amyl acetate. Shake thoroughly and quickly(since U-X

2has a very short half-life) to extract U-X2 complex

into the amyl acetate layer, Allow to settle, then the bottomlayer (aqueous layer) is quickly separed, the organic layer is runslowly into a petri dish (see the setting of the apparatus fromExpt. C.2.1) and is monitored for the decay of U-X2 in the samemanner as described in experiment C.2.1.

In place of amyl acetate, other solvents mentioned prior tocommencing of this experiment can be used with slight modificationfor example the extraction of U-X2 from a hydrochloric acid solu-tion of uranyl nitrate into isobutyl ketone should yield a similarresult to that discussed above.

*Broadbank R.W.C, and S. Dhabanandana, Letter to editor, J. Chem.Ed., Vol. 37, No. 3, (1960).

- 33 -

C.4 Precipitation

Thorium-B (Pb212

) and thorium-C (Bi212

) are consecutive mem-bers of the thorium disintegration series.

0aTh

232 > Ra228---0, Ac 228 0--------> Th

228

1.39x10 10yrs. 6.7 yrs, 6.13 h.

1.9 yrs

pb212a 1,02164a___Rn

2204---1---- Ra

224

(Th-B) 0.16 sec. 54.5 sec. 3.614 d.

1

0 10.6 h.

Bi212 a T1

208 B Pb208(Th-C) 3.i min.

/$ 60.5 min.

They will therefore be present in thorium compounds which have notbeen treater'. chemically for several months and may be separated bysuch methods as soAygnt extrgqion or precipitation. In the preci-pitation method Pb`'` and Bi are precipitated as lead sulphideand bismuth sulphide (isotopic carriers are added) by hydrogensulphide from acid solution. Advantage of the insolubility of leadsulphate is taken to separate lead from bismuth. The growth curveof B1212 may be observed in the precipitate of lead sulphate (takesapproximately 4 hours to grow to a maximum thereafter it will decaywith the half life of Pb212'). The decay of Bi212 (Th-C) is followedin the solution of sulphate of bismuth.

Experiment C.4.1 Separation of Th-B and Th-C by Precipitation

Material Required

Thorium nitrate2M hydrochloric acid1.5%bismuth chloride solution1.5% lead nitrate solutionHydrogen sulphide gasG.M.-counter with a scaling unit or a rate meter

- 314 -

Procedure

A dilute acid solution of thorium nitrate is prepared by ditl-solving approximately 10 grams of thorium nitrate in a little wateirand a few drops of 2M hydrochloric acid are added. Approximately1 ml of 1.5%solution of lead nitrate and 0.5 ml of 1,5% solutionof bismuth chloride are added as carriers of Th-E and Th-C. Hydro-gen sulphide is.passed into a hot solution of thorium nitrate untilprecipitation of sulphide is complete. The precipitate is washedwith warm 1% hydrochloric acid to free it from thorium salts. Thewashings being separated by centrifuging and discarded. The sul-phides are separated afterwards by dissolution in dilute nitricacid followed by precipitation of lead sulphate (carrying lead-212Or Th -B) having a little concentrated sulphuric acid as a precipi-tating agent. The tube is left in a beaker of almost boiling water.After evolution of nitrous oxide fumes has ceased, it is cooledand diluted to approximately 10 ml with water and centrifuged.The time is now noted and the solution x which contains Th-0 acti-vity is set aside while the precipitate of lead sulphate is washedwith 5% sulphuric acid, water and acetone successively and is madea slurry with small amount of acetone and transferred to a source.tray, is dried and covered with "Cellotape". The source is countedfor activity of Th-C which grow from zero to a maximum every halfan hour. The activity due to Th-B is screened with a piece ofaluminium absorber of about 250 mg/cm` placed between the sourceand detector (G.M.-tube). Growth of Th-C in Th-E should be ob-served.

The half life of Th-C (60.5 minutes) can be determined fromsolution x, employing the same procedure as described in experi-ment C.2.1.

Th-C decays by an a emission to thallium-208 which is a 6-emitter with hAlf life of 3.1 lilptes. The study of half-life canbe made on Ti "' instead of El' . The solution x which containsoluble sulphate of bismuth is poured through a small column ofthe hydroxide form of "Deacedite FF" or similar (strong anion ex-change resin). Bismuth is held very stumgly under these condi-tions and after 10 minutes or so the Tl.'"(1 which has grown may bewashed out with very dilute hydrochloric acid into a petri dishand is monitored for the decay of T120 and hence its half-lifeis obtained.

The above experiment has been developed at the RadiochemistrySection, Leicester Regional College of Technology, England.

-`4" ,110 ks

, -

). ."'

- 35 -

piscussions and Conclusions

Short lived nuclides are separated from their parents by sim-ple methods and this provides a useful demonstration of radioactiVegrowth and decay.

The experiments present in this section except experimentsF.1.1, C.1.2 and C.h.1 yield carrier free tracers, Whenever carrieris added, the amount of carrier is kept as small as possible,usually about 5-50 mg in order to minimize measurement problems.

There is no radiation hazard in any experiments if the amountof radioactive material is kept to the minimum necessary for satis-1

Vactory measurement. Nevertheless, it should be made a habit noto use mouth operation, not to pick a source of activity up with abare hand and always use a pair of tweezer to pick a solid sourcepnd wrap a beaker or tube etc. containing radioactive substances,fith tissue paper before holding by hand. Uranium solutions aretoxic. There is no danger to health arising from the radioactivesubstances but there is appreciable risk of contaminating thecounting apparatus.

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3A

Aims and Objectives of this Project

The General Conference of Unesco, at its thirteenth session, adoptedResolution 2.122 to organize a Pilot Project for Chemistry Teaching in Asiafor the purpose of initiating a fundamental re-orientation in the way of teachingchemistry through the use of modern technical devices and methodology. Anagreement was signed between the Government of Thailand and Unesco on13 July 1965 to locate the Project at Chulalongkorn University, Bangkok. TheProject started as a regional project. In addition to its regional activities, theProject centre has increasingly served as a national centre for Thai scienceeducation.

The primary aim of the Project is to assist science educators in Asia intheir task of carrying out reform of chemistry teaching. The Project is opera-ting along two major lines which are distinct but co-ordinated:

1. Modernization of the chemistry courses and development of new teachingmaterials.

2. Assistance in carrying out in-service and pre-service teacher training, im-provement of examinations and use of the latest methods of teaching.

Science educators in Asia may wish to request some or all of thefollowing resource materials (at no cost) in sample quantities to help them carryout curriculum reform :

1. Programmed Instruction Sequence, 1966.2. Teachers' Guide to the above programmed sequence, 1966.3. 8 mm. Film Loops in Cassettes.*4. Film Loop Production Notes, 1967.5. Teachers' Guide to Film Loops, 19676. Compound Formation (Vols, I and II), Teachers' Digest, 1967, 1968.7. Chemical Equilibria, A Teachers' Digest, 1968.8. Experiments on Chemical Equilibria, 1968.9. Experiments on Compound Formation, 1969.

10. Compound Formation Vol. 1 (Thai translation), 1969.11. Experiments on Chemical Equilibria (Thai translation), 1969.12. Newsletter, a bi-monthly periodical.13. Prototypes of low cost kits: "Teaching Experiments on Chemical

Equilibria", "Teaching Experiments on Compound Formation" and"Teaching Experiments on Rate of Chemical Reactions."

14. Experiments on Rate of Chemical Reactions, 1969.15. Experiments on Nuclear Science, 1969.

Available at a cost of U.S. 6.00 per film loop." Cannot be supplied outside Thailand.